Conventional dimmers for operating magnetically ballasted discharge lamps typically utilise an approach of applying of phase control to AC line voltage. Equipment or circuitries incorporating this approach often couples a triac, or paired silicon controlled rectifier (SCR), in series with a lighting fixture. By delaying switching on of a switch at a phase angle from the zero crossing of the AC line voltage, AC power communicated to the lighting fixture is increased or decreased to thereby control intensity of a discharge lamp. FIG. 1 shows exemplary AC current half-cycles across the discharge lamp when phase control is applied by a typical dimmer system to the AC power communicated thereto.
The main problem associated with the phase control approach for varying the AC power communicated to the discharge lamp is that this often results in occurrence of flicker when the AC power communicated to the discharge lamp is reduced during dimming. This becomes progressively worse as AC power level drops below 70% and discontinuity in AC current across the gas discharge lamp increases. The discontinuity in the AC current across the discharge lamp can lead even to the gas discharge lamp being extinguished.
Therefore, there exists a need for an improved dimmer and an improved dimming method.